Technical Field
The present invention relates to a lithium ion cell. More specifically, the present invention relates to a lithium ion cell having a power generation part provided with a single cell obtained by stacking a positive electrode current collector, a positive electrode active material layer, a separator, a negative electrode active material layer, and a negative electrode current collector in the order, in which the single cell has flexibility.
Related Arts
The lithium ion cell is a small-sized and light-weighted secondary cell having large capacity, and it is recently used for various applications. In accordance with a trend of having a smaller size or better performance of an electronic device in which a lithium ion cell is used as a power source, currently there is a demand for having a lithium ion cell with flexibility and large capacity.
As a cell with flexibility, a thin film type cell using a specific film exterior body (JP 2013-48041 A) and a lithium ion cell using a softening agent in an active material layer (WO 2014/208675 A) are known.
Furthermore, as a cell capable of exhibiting high power density, a stack type bipolar cell having a cell element, in which a single cell layer obtained by stacking a positive electrode current collector, a positive electrode active material layer, an electrolyte layer, a negative electrode active material layer, and a negative electrode current collector in the order is stacked serially, an exterior material for wrapping and sealing the entire cell element, and an electrode terminal drawn from the exterior material to the outside for drawing electric current is known (JP 2004-158306 A).
The bipolar cell means a cell in which a plurality of single cells are stacked and connected serially, and it has a structure in which the positive electrode current collector and negative electrode current collector are stacked either directly or via other member while they are in a state of maintaining electric connectivity. Since the electric current flows in the thickness direction, the bipolar cell has a short current path. Accordingly, the loss of electric current is small and a decrease in power is low, and thus it is possible to increase the power density, and when the amount of an active material contained in a cell is increased by increasing the stacking number of a single cell, it is also possible to achieve large capacity.
However, according to a cell with flexibility of a related art described in the above, the active material layer has flexibility, and thus there is a possibility that, when the lithium ion cell is significantly deformed, gaps or the like may occur within the active material layer and sufficient charge and discharge characteristics may not be exhibited as the conductive path is blocked.
Furthermore, for having large capacity, it is efficient to increase the amount of an electrode active material. However, according to the techniques of a related art, it is necessary to prepare a lithium ion cell in a thin film form to maintain the flexibility so that it is difficult to increase the amount of an electrode active material which can be used, that is, a thick electrode active material layer cannot be formed, and thus it is difficult to have large capacity while maintaining the flexibility.
Furthermore, as the stack type bipolar cell of a related art described above has insufficient active material layer flexibility, there is a possibility that, when a stress is applied to a main body of cell, defects like gap may occur due to a deformation occurring within the active material layer and sufficient charge and discharge characteristics may not be exhibited as the conductive path is blocked, and thus it is difficult to have simultaneously the cell characteristics such as high power density or large capacity and the mechanical durability.
The present invention is devised in view of the problems described above, and one object of the invention is to provide a lithium ion cell capable of having both the flexibility and large capacity so that sufficient charge and discharge characteristics can be exhibited even when the lithium ion cell is significantly deformed, and also to provide a stack type lithium ion cell capable of having both the cell characteristics such as high power density or large capacity and the mechanical durability.